One quote in particular "it is still not known whether the daytime nap itself, a supine posture, or the expectancy of a nap is the most important factor".
If you take a nap at the same time every day the expectancy reward is....so so. If you take a nap whenever you feel like it then you may benefit more (not included in the study). I generally nap after eating, it gives the enteric nervous system (our other brain) all my energy to do its thing.